Treatment of hydrocarbon oils



Nov. 21, 1933. G. ARMISTEAD, JR 9 2 TREATMENT OF HYDROCARBON OILS Filed March 25, 1931 Fefic T/a/vfi TING Ton [e FLASH ST/LL $0265 Del/M /%M/7Q/ INVENTOR BY fl Am A TTORNE Y pressure.

Patented Nov. 21, 1933 UNITED srATEs PATENT. OFFICE The Texas Company, poration of Delaware Application March 25,

3 Claims.

This invention relates to conversion of higher boiling hydro-carbon oils into lower boiling hydrocarbon oils and has par-"cular reference to the treatment of the residual liquid from a cracking system to efiect vaporization thereof and the subsequent fractionation of the evolved vapors.

The invention involves a process wherein the tar, residuum or unvaporized oil is withdrawn from a conversion system, the pressure reduced whereby vaporization occurs as a result of the latent or self-contained heat of the liquid, the vapors fractionated to form a naphtha vapor fraction and a reflux condensate, consisting chief ly of gas oil, and the reflux condensate in part or entirely conducted to a stripping chamber to remove any low boiling naphtha constituents. The invention also includes a novel apparatus consisting of a combined tar stripper and an im proved fractionating tower which provide certain features of compactness and efficiency that will be more fully described hereinafter.

The usual method of treating pressure tar is to withdraw the tar into a flash drum maintained under a reduced superatmospheric pressure wherein vaporization occurs. The vapors are fractionated to produce a lighter naphtha fraction and a heavier gas oil fraction which is recycled to the cracking still. The pressure commonly used on the flash drum is not less than 10 to 15 pounds gauge which has been the minimum that could safely be employed due to the tendency of the tar to surge as it is drawn oif and thereby cause priming of the vaporizer and fractionating equipment. As a result of the substantial superatmospheric pressure on the flash still, it has been necessary to use large quantities of steam to obtain the desired amount of distillation. This steam has materially increased the cost of operation.

A feature of the present invention is the provision of a combination of a surge drum and vaporizer whereby it is possible to reducethe pressure on the vapor zer substantially below 15 pounds gauge and preferably near atmospheric The low pressure is possible because of the absorption of the surges or pressure fluctuations by the surge drum so that a relatively low pressure may be maintained on the flash drum and at the same time an even evolution of vapors may be efiected without the danger of priming. The low pressure of the flash drum or vaporizer has a distinct advantage in that the desired degree of vaporization may be obtained without the use of steam which is not only expensive in itself but also increases the amount of cooling required to condense the vapors.

In the priormethods of flash distillation of residuum as heretofore described, the cycle gas oil or reflux condensate from the tar stripper ordinarily contains substantial quantities of low New York, N. Y., a cor- 1931. Serial No. 525,084

boiling naphtha. The presence of this naphtha in the cycle gas oil charged to the cracking system reduces the amount of charge corresponding to the naphtha present and also reduces the yield of gasoline on the basis of the crackable material present in the charge. The presence of lighter fractions such as naphtha inthe charge furthermore retards cracking since such constituents tend to lower the still temperature as a result of their vaporization.

Another feature therefore of the present invention is the elimination of the naphtha from the cycle gas oil. This is done by an auxiliary stripping operation. The reflux condensate in whole, or in part as a side stream, is conducted to a stripping chamber where the last traces of naphtha are removed. In this Way it is possible to produce cycle gas oil which is free from naphtha as evidenced by an initial boiling point above the end point of the light naphtha fraction.

Thus, if a naphtha is being produced with a maximum boiling point of 400 F., a cycle gas oil may be produced at the same time with an initial boiling point above 400 F.

As a result of the efficiency of the present invention whereby the naphtha and cycle gas oil are closely cut so that there is no over-lapping of the boiling points of the various fractions, an extraneous oil may be used as a reflux to strip out the naphtha content thereof. In this way products which have been made on less efficient equipment or gas oils which have been contaminated with lower boiling constituents during handling may be conveniently usedas arefiux medium. The advantages of the. inventionwill be further observed from a description of the drawing which illustrates an embodiment thereof.

In'the drawing thereference characterl represents a surge'drum, 2 a flash still or'vaporizing chamber and 3 a fractionating tower. Aline/l serves to conduct unvaporized residual liquid or tar from the cracking system to the surge drum. A connection 5 is adapted to convey liquid from the surge drum to the flash still. A valve 6 is interposed in the line 5 to reduce the pressure on the products passing therethrough.

The flash still 2 is equipped with suitable baffling material 7 to knock out entrained liquid particles from the vapors. A residue line 8 communicates with the bottom of the flash still and a pump 9 interposed therein, withdraws funvaporized liquid from the flash still .to a cooling coillil. A vapor line 15 conducts vapors from the flash still to the bubble tower.

The tower 3 is preferably made in two sections which are blanked off from each other by a. partition 16. Each section is provided with suitable bubble trays 17. The upper section constitutes a fractionator or bubble tower, the vapors entering near the bottom thereof through the vapor line 15. A vapor line 18 serves to conduct vapors from the top of the bubble tower to a condenser coil 19. A line 20 conducts condensate from the condenser 19 to a water separator 21 which is equipped with a drain pipe 22. A pipe 23 connects the separator 21 with a receiver 24, the latter being provided with the usual gas release pipe 25 and a liquid draw-off line 26. Pumps 27 and 28 serve to pump the liquid from receiver 24 to storage through the line 29, or, if desired, back to the bubble tower through the line 30 to serve as a reflux medium. Branches 31 and 32 connected to the pipe 3i) serve to conduct the condensate to either or both of the two top trays of the fractionating tower by suitable regulation of valves interposed therein. A pump 35 serves-to draw an extraneous reflux medium through the line 36 and by suitable regulation of valves in the branches 3'7, 38, 39 this reflux medium may be passed to any or all of three upper trays, preferably the 4th, 5th, and 6th trays.

The lower section of the tower 3 below the partition 16 represents the stripping section or stripping tower. This section is in communication witlrthe upper section through a vapor line 40 and a liquid line 41. The liquid line 41 serves to draw an intermediate out or side stream from the bubble tower and by means of branches 42, 43, 44 a liquid may be drawn from anyone or all of three intermediate trays of the bubble section and passed to the stripping section.

A by-pass line 45 maybe used to conduct any or all of the reflux condensate collecting in the bottom of the upper section to the line 41 whereby this reflux condensate may also be passed to the lower section. The line 46, term nating in a spray 47 serves to admit steam to the bottom of the stripping section. Liquid draw-off lines 50 and 51 in which are interposed pumps 52 and 53 serve to conduct the condensates collecting'in the bottoms of the upper and lower sections respectively to a cooler 55, the latter havinga plurality of coils therein. The lines 50;.and 5'lconnect to a manifold 57 so that anydesir'ed number of cooling coils may be employed. A second manifold 58 at the outlet of the cooling coils serves to conduct the. cool condensates to the proper discharge pipes 59 or60.

While the tower 3 is shown as divided into two sections with the bubble tower superimposed on 'th-e';stripper,'it is to be understood that the two secaons may be separated "into two distinct towjars-or chambers. However, I prefer the arrangement'as shown in the drawing since it is a compact arrangement which considerably reduces the investment cost.

In practicing the invention with an apparatus such as that shown in the drawing, the residual liquid, pressure tar or residuum resulting from the conversion of hydrocarbon oils, is drawn through the "line 4 which may communicate with "the reaction chamber or vaporizer of any well known or preferred type of conversion system. The hot liquid is passed to a surge drum where japool of the liquid is maintained at approximately the same pressure as is maintained on the conversion system; The surge drum is built to withstand a high pressure and its function is to absorb the surges or fluctuations in pressure "so that an even stream of liquid under uniform pressure be withdrawn therefrom through the pipe 5. The pressure is released at the valve 6 whereby vaporization of the liquid is effected in the flash still 2. When the pressure is released, a sudden vaporization occurs due to the latent or self contained heat of the liquid.

By the use of a surge drum and the consequent elimination of surges as the residue is withdrawn, it is possible to maintain at a minimum, the pressure on the flash drum 2. A low pressure atmospheric maintained on the flash drum points the desired vaporization without the aid of steam. A heavy fuel oil is drawn through the line 3 while the vapors consisting of lighter products, including gas oil, pass through the line 15 to the bubble tower. The vapors enter bubble tower below the bottom tray and pass up through success. :e pools of oil to effect fractionation. A vapor fraction consisting of naphtha, preferably about 490 F. endpoint, is taken off from the top the tower through the line 18 and passed to the condenser 19. The water condensing with the naphtha may be separated in a separator or trap 21 and the condensate collected in a receiver 24. A part of this condensate is preferably recycled to the two top trays of the bubble tower to act as a reflux to control the endpoint of the distillate.

To provide sufficient refluxing, a cool, extraneous oil may be pumped to the 4th, 5th, or 6th trays through the line 36 and the branches 37, is often convenient to use for this reflux medium a product that has been insufficiently fractionated; that is, one containing an appr eiable quantity of naphtha such as gas oil, or cycle stock from which it is desirable to remove the naphtha content before charging to the cracking still.

The fractionating tower may be operated in either of two ways. In one method of operation, the reflux collecting in pools on the trays, overflows from tray to tray and accumulates at the bottom of the upper section just above the partition 16, This single cut consisting chiefly of gas oil is then drawn oil through the line 50 and may be passed to the cooler or any or all of it may be by-passed through the line 45 and pipe 41 to the lower stripping section.

The preferred method of operating the fractionating tower is to make two outs of gas oil, the lightest one of which is taken as aside stream from one or more of the intermediate trays of the bubble tower, the-amount of this side stream being regulated by valves at the side of the tower in the branches 42, 43 and 44. If 'desired, however, both methods may be used simultaneously by suitable regulation of valves 42,, 43 and 44 and the valve in the by-pass line 45.

In either method of operation, one stream of gas oil requires steaming to remove a small naphtha content. To do this-a stripping section, having preferably four trays, is provided. The side stream from the intermediate trays, or the total gas oil out from the bottom of the bubble tower, according to the method of operation, is passed through the line 41 to the top of the stripping section. In this section, the oil passes down over the trays, thereby effecting separation of light fractions and stripped oil collects in the bottom of the section, where the liquid is further stripped by steam introduced through the spray pipe 47.

The gas oil is withdrawn from the bottom of the stripping section through the line 51 and pumped to the cooler. This gas oil is-entirely free from naphtha content and the initial boiling point is usually higher than the endpoint'of the naphtha recovered in the receiver 24. In other words, if the naphtha in the receiver 24 has an endpoint of 400 F., the initial boiling point of the gas oil withdrawn from the bottom of the stripping section would be in excess of 400 F.

It will be observed, therefore, that the apparatus is flexible and provides a novel means for not only removing entirely the naphtha from the gas oil but also for efiecting segregation of certain gas oil cuts, if desired.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. The method of treating a hot-residual tar resulting from the conversion of hydrocarbon oils which comprises subjecting said tar to flash distillation to vaporize the low boiling constituents thereof, subjecting the resultant vapors to fractionation in a fractionating zone to separate the naphtha in the form of vapors and to condense the gas oil fraction as a reflux condensate, refluxing said resultant vapors at an intermediate point in said zone with gas oil containing naphtha, refractionating said gas oil fraction to strip therefrom remaining naphtha, and mixing said remaining naphtha with the vapors in said fractionating zone.

2. The method of treating hot-unvaporized residue resulting from the conversion of higher boiling hydrocarbon oils into lower boiling hydrocarbon oils which comprises withdrawing said residue into a surge drum, passing the residue from the surge drum with reduction of pressure to a vaporizing zone wherein vaporization is effected by the self-contained heat of said residue, passing the evolved vapors to a fractionating zone to form a naphtha vapor fraction and a reflux condensate, refluxing said resultant vapors at an intermediate point in said zone with gas oil containing naphtha, withdrawing said reflux condensate from the fractionating zone in the form of a plurality of fractions of different boiling point, passing at least one of said fractions to a stripping zone wherein the lighter fractions thereof are removed as vapors and passing said vapors to the fractionating zone.

3. The method of treating hot tar derived from a pressure cracking operation which com prises introducing said tar into a zone of reduced pressure wherein vaporization oc'curs, removing vapors from said zone and introducing them into a low point in a fractionating zone wherein partial condensation occurs, introducing into an intermediate point in said fractionating zone, as a refluxing medium, gas oil containing naphtha, supplying cooling to the top of said zone, removing reflux condensate from a point in said fractionating zone below said intermediate point, separately stripping said condensate of light naphtha vapors, returning said light naphtha vapors to said fractionating zone and removing uncondensed vapors of the desired characteristics from an upper point in said fractionating zone.

GEORGE ARMISTEAD, JR. 

